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| /openbmc/linux/fs/btrfs/ |
| H A D | relocation.c | f7ba2d37 Wed Dec 16 10:22:15 CST 2020 Josef Bacik <josef@toxicpanda.com> btrfs: keep track of the root owner for relocation reads
While testing the error paths in relocation, I hit the following lockdep
splat:
======================================================
WARNING: possible circular locking dependency detected
5.10.0-rc3+ #206 Not tainted
------------------------------------------------------
btrfs-balance/1571 is trying to acquire lock:
ffff8cdbcc8f77d0 (&head_ref->mutex){+.+.}-{3:3}, at: btrfs_lookup_extent_info+0x156/0x3b0
but task is already holding lock:
ffff8cdbc54adbf8 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_lock+0x27/0x100
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (btrfs-tree-00){++++}-{3:3}:
down_write_nested+0x43/0x80
__btrfs_tree_lock+0x27/0x100
btrfs_search_slot+0x248/0x890
relocate_tree_blocks+0x490/0x650
relocate_block_group+0x1ba/0x5d0
kretprobe_trampoline+0x0/0x50
-> #1 (btrfs-csum-01){++++}-{3:3}:
down_read_nested+0x43/0x130
__btrfs_tree_read_lock+0x27/0x100
btrfs_read_lock_root_node+0x31/0x40
btrfs_search_slot+0x5ab/0x890
btrfs_del_csums+0x10b/0x3c0
__btrfs_free_extent+0x49d/0x8e0
__btrfs_run_delayed_refs+0x283/0x11f0
btrfs_run_delayed_refs+0x86/0x220
btrfs_start_dirty_block_groups+0x2ba/0x520
kretprobe_trampoline+0x0/0x50
-> #0 (&head_ref->mutex){+.+.}-{3:3}:
__lock_acquire+0x1167/0x2150
lock_acquire+0x116/0x3e0
__mutex_lock+0x7e/0x7b0
btrfs_lookup_extent_info+0x156/0x3b0
walk_down_proc+0x1c3/0x280
walk_down_tree+0x64/0xe0
btrfs_drop_subtree+0x182/0x260
do_relocation+0x52e/0x660
relocate_tree_blocks+0x2ae/0x650
relocate_block_group+0x1ba/0x5d0
kretprobe_trampoline+0x0/0x50
other info that might help us debug this:
Chain exists of:
&head_ref->mutex --> btrfs-csum-01 --> btrfs-tree-00
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(btrfs-tree-00);
lock(btrfs-csum-01);
lock(btrfs-tree-00);
lock(&head_ref->mutex);
*** DEADLOCK ***
5 locks held by btrfs-balance/1571:
#0: ffff8cdb89749ff8 (&fs_info->delete_unused_bgs_mutex){+.+.}-{3:3}, at: btrfs_balance+0x563/0xf40
#1: ffff8cdb89748838 (&fs_info->cleaner_mutex){+.+.}-{3:3}, at: btrfs_relocate_block_group+0x156/0x300
#2: ffff8cdbc2c16650 (sb_internal#2){.+.+}-{0:0}, at: start_transaction+0x413/0x5c0
#3: ffff8cdbc135f538 (btrfs-treloc-01){+.+.}-{3:3}, at: __btrfs_tree_lock+0x27/0x100
#4: ffff8cdbc54adbf8 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_lock+0x27/0x100
stack backtrace:
CPU: 1 PID: 1571 Comm: btrfs-balance Not tainted 5.10.0-rc3+ #206
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
Call Trace:
dump_stack+0x8b/0xb0
check_noncircular+0xcf/0xf0
? trace_call_bpf+0x139/0x260
__lock_acquire+0x1167/0x2150
lock_acquire+0x116/0x3e0
? btrfs_lookup_extent_info+0x156/0x3b0
__mutex_lock+0x7e/0x7b0
? btrfs_lookup_extent_info+0x156/0x3b0
? btrfs_lookup_extent_info+0x156/0x3b0
? release_extent_buffer+0x124/0x170
? _raw_spin_unlock+0x1f/0x30
? release_extent_buffer+0x124/0x170
btrfs_lookup_extent_info+0x156/0x3b0
walk_down_proc+0x1c3/0x280
walk_down_tree+0x64/0xe0
btrfs_drop_subtree+0x182/0x260
do_relocation+0x52e/0x660
relocate_tree_blocks+0x2ae/0x650
? add_tree_block+0x149/0x1b0
relocate_block_group+0x1ba/0x5d0
elfcorehdr_read+0x40/0x40
? elfcorehdr_read+0x40/0x40
? btrfs_balance+0x796/0xf40
? __kthread_parkme+0x66/0x90
? btrfs_balance+0xf40/0xf40
? balance_kthread+0x37/0x50
? kthread+0x137/0x150
? __kthread_bind_mask+0x60/0x60
? ret_from_fork+0x1f/0x30
As you can see this is bogus, we never take another tree's lock under
the csum lock. This happens because sometimes we have to read tree
blocks from disk without knowing which root they belong to during
relocation. We defaulted to an owner of 0, which translates to an fs
tree. This is fine as all fs trees have the same class, but obviously
isn't fine if the block belongs to a COW only tree.
Thankfully COW only trees only have their owners root as a reference to
them, and since we already look up the extent information during
relocation, go ahead and check and see if this block might belong to a
COW only tree, and if so save the owner in the tree_block struct. This
allows us to read_tree_block with the proper owner, which gets rid of
this lockdep splat.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
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